The Third Generation Partnership Project (3GPP) Long Term Evolution (LTE) standards propose using an Orthogonal Frequency Division Multiple Access (OFDMA) for transmission of data over an air interface. In an OFDMA communication system, a frequency bandwidth is split into multiple contiguous frequency sub-carriers, wherein groups of sub-carriers are arranged in logical frequency resource blocks (not necessarily contiguous in frequency), each resource block comprising multiple orthogonal frequency sub-carriers, that are transmitted simultaneously. A user may then be assigned one or more of the frequency resource blocks for an exchange of user information, thereby permitting multiple users to transmit simultaneously on the different resource blocks. These resource blocks are orthogonal to each other, and thus inter-user and intra-cell interference is minimized.
In order to provide more efficient use of the channel bandwidth, a radio access network (RAN) may transmit the data using multiple antennas and a user equipment (UE) may receive the transmitted data using multiple receiving antennas, referred to as Multiple Input-Multiple Output (MIMO). In an OFDMA system that implements MIMO, a serving RAN may beamform a downlink signal for transmission to each UE via an antenna array and over an associated resource block. In order to beamform the signal, the RAN maintains a set of (transmit) weights in association with each UE and each element of the antenna array. When the RAN transmits to the UE, the RAN applies an appropriate weight, of the set of weights, to the signal applied to each element of the array. In order to determine the set of weights for each UE, the RAN measures uplink channel conditions in association with the UE. That is, for any given measuring period, such as a Transmission Time Interval (TTI) (also known as a sub-frame), a UE served by the RAN transmits a pre-determined symbol to the RAN in an uplink sounding interval, or sounding zone, that is reserved for the transmission of sounding signals. Based on a comparison of the symbol received to the symbol that the RAN knows was transmitted, the RAN is able to estimate channel conditions associated with the UE and determine a set of weights for a downlink transmission to the UE. The transmission of sounding signals can also used to support the scheduler to perform frequency selective scheduling of uplink data transmissions (applicable for both Time Division Duplex (TDD) and Frequency Division Duplex (FDD) scenarios).
For example, FIG. 1 is a block diagram 100 depicting a channel sounding for a TDD system of a frequency bandwidth 102 in accordance with the prior art. As depicted in FIG. 1, during a first transmission interval 104 a RAN transmits a first downlink (DL) sub-frame 110. During a next, second transmission interval 106 a UE served by the RAN transmits an uplink (UL) sub-frame 120 to the RAN, and during a next, third transmission interval 108 the RAN transmits a second DL sub-frame 130. In an FDD system, the frequency location of the DL and UL subframes are different, and the frequency bandwith 102 may be different as well.
Each DL sub-frame 110, 130 includes a control region 112, 132 and a DL data packet field 114, 134. Each DL sub-frame 110, 130 further may include reference signals that may be used by UEs for timing synchronization, frequency synchronization, and channel estimation. The control region 112, 132 contains several downlink control channels—the Physical Downlink Control Channel (PDCCH), the Physical Control Format Indicator Channel (PCFICH), and the Physical H-ARQ Indicator Channel (PHICH). The PCFICH provides information about the size of the control region. The PHICH provides H-ARQ acknowledgements for uplink data transmission. The PDCCH provides uplink and downlink scheduling assignment allocation, and power control commands. The downlink scheduling assignment allocation provides a subband allocation on the DL subframe, HARQ process number and coding and modulation scheme which is specific to a particular user. In addition, precoding information may also be provided. Similarly, the uplink scheduling assignment allocation provides a subband allocation on the UL subframe and coding and modulation scheme which is specific to a particular user.
UL sub-frame 120 includes an UL data packet field 122 and a sounding zone 124. UL data packet field 122 comprises UL bursts, that is, is the field in which the UEs transmit data packets to the RAN based on UL scheduling field 116. Sounding zone 124 is a field in which each of one or more UEs served the RAN transmits, over the frequency banmdwidth, a predetermined Sounding Reference Signal (SRS) known to both the RAN and the UE. In TDD, channel sounding assumes a reciprocity of the UL and DL channels and also assumes the RAN has a means of accounting for any non-reciprocities that may exist in the RAN transceiver hardware. Based in the received SRS, the RAN is able to estimate a RAN-to-UE channel response, adaptively schedule a resource block comprising a set of sub-carriers for a DL transmission to the UE, and adaptively determine a set of weights for the DL transmission to the UE over the scheduled set of sub-carriers. The RAN then conveys a DL burst 140 to the UE in a DL data packet field 138 of DL sub-frame 130 transmitted during the next time interval 108. The DL burst is transmitted over the scheduled set of sub-carriers and resource block using the set of weights determined based on the received SRS.
In addition to weight determination, the SRS can also be used for frequency selective scheduling in the uplink. Based on the transmitted SRS, the RAN measures the channel response in each of the resource block. When UL data transmission is scheduled, then RAN can then use information about each of the resource block in the resource block allocation.
Currently, under the 3GPP LTE standards, UEs are preconfigured to periodically transmit SRSs in the uplink. The SRS configuration is semi-static, for example, the UEs are preconfigured to transmit the SRS at a predetermined time and frequency using a predetermined orthogonal code, and accordingly only a limited number of SRSs, and correspondingly only a limited number of UEs, can be concurrently supported by a channel sounding-based system. In order to increase system capacity, a trigger-based SRS has been proposed for 3GPP LTE-A (Long Term Evolution-Advanced). However, no mechanism has been proposed for implementation of a trigger-based SRS.
Accordingly, a need exists for a method and apparatus for implementing a trigger-based SRS in an OFDMA communication system.